1. Field of the Invention
This invention relates to a semiconductor device, and particularly to a light emitting device disposed on a semiconductor monocrystalline substrate by way of a heterojunction.
2. Related Arts
Conventionally, in a discrete ordinary light emitting diode of the kind seen in a GaAs light emitting diode made by homoepitaxial growth on a GaAs substrate (that is, a GaAs light emitting diode wherein an n-GaAs layer and a p-GaAs layer are consecutively formed by crystal growth on a GaAs substrate and an upper electrode is locally disposed on the upper surface of the GaAs layers), to improve the light output efficiency the following kinds of method have been being used: (1) To cause current to flow through a large region of a pn junction surface and cause light to be emitted from a large region of the pn junction surface, the film thickness of operating layers of the diode are made large and the electrical field inside the diode is extended in the transverse direction. (2) To avoid blocking of light by an upper electrode, the upper electrode and a lower electrode of the diode are disposed offset from each other, current in operating layers is made to flow diagonally and a pn junction portion directly below the upper electrode is not made a light emitting region (for example the structure disclosed in Japanese Laid-Open Patent Publication No. H5-90638).
However, when this technology is used in a light emitting diode in which a monolithic structure has been adopted with the purpose of integrating constant voltage circuits and signal processing circuits and the like, that is, when a light emitting diode is formed by consecutively forming an n-GaAs layer and a p-GaAs layer on an Si substrate by crystal growth, it is known that when a GaAs crystal of over 5 .mu.m is grown, because the thermal expansivities of Si and GaAs are different, cracks occur (Autumn 1987, Applied Physics Society Advance Articles page 236, 20p-X-13; Characterization of GaAs Layer Grown on Si Substrates by MBE; Ishino et al. ). Consequently, it is necessary to keep the thickness of a GaAs light emitting diode (the thickness of the n-GaAs layer and the p-GaAs layer) below 5 .mu.m. However, in a light emitting diode made with a normal film thickness, current only flows substantially directly below the electrode. That is, in a light emitting diode in which an upper electrode is locally disposed on the upper surface of a GaAs layer, because current only flows directly below the upper electrode it is necessary for light to be outputted through the upper electrode and the efficiency with which the light is outputted to the outside has consequently been low.
Also, with this structure (a structure comprising a laminate of an n-GaAs layer and a p-GaAs layer on an Si substrate), it is not possible to form a lower electrode directly on the diode (the laminate of GaAs layers), and consequently it is not possible to output light to the outside without suffering a blocking influence (influence attenuating the light) caused by the upper electrode by offsetting the positions of the upper electrode and the lower electrode.
As technology devised in view of this problem, there is a light emitting diode disclosed in Japanese Laid-Open Patent Publication No. H3-283676. This has a current blocking layer disposed between a double-hetero structure part and a substrate for preventing current from flowing to a region located below an upper first electrode.
That is, when an n-type InGaAlP cladding layer and a p-type current blocking layer are joined, a depletion layer determined by a diffusion potential and impurity concentrations of the two layers is formed at their interface. The current blocking layer blocks current by utilizing a pn diode constituted by these two layers becoming reverse biased.
However, in the light emitting diode disclosed in Japanese Laid-Open Patent Publication No. H3-283676, the impurity concentration of the n-type InGaAlP cladding layer is set high with the object of injecting carriers. Consequently, the depletion layer formed between the n-type InGaAlP cladding layer and the current blocking layer does not readily extend to the n-type InGaAlP cladding layer side, and there is the problem that it is not possible efficiently to block current flowing within the device.
Here, to increase the spread of the depletion layer to the n-type InGaAlP cladding layer side, it is conceivable to make the impurity concentration of the current blocking layer high also; however, in this case, the tunnel effect becomes great and as a result it is not possible to block current efficiently.